Color television systems have been developed using several different broadcast and signal processing formats to achieve the successful transmission and reception of color television programming. While substantial differences between systems exist, they all must satisfy the basic objective of combining the picture or luminance information, the color or chrominance information, and sound information together with appropriate display scan synchronizing signals to form an information signal which may be modulated upon a carrier for transmission. At the receiver, the opposite processes must take place in which the several components of the information signal are separated and appropriately processed. In most television broadcast formats such as the NTSC system used within the United States of America and the PAL system used in many European countries, the signal components corresponding to luminance, chrominance and sound are distinguished from each other and separated for individual processing largely on the basis of signal frequencies. For example, in the NTSC system, the available broadcast bandwidth is maintained at 6 megahertz. To conserve channel bandwidth and still transmit a video signal up to 4.1 megahertz, a vestigial sideband format in which the carrier is off center within the 6 megahertz channel bandwidth is used. The chrominance information is modulated upon a chrominance subcarrier separated from the picture carrier by approximately 3.58 megahertz. The sound information is separated from the picture carrier by 4.5 megahertz.
Thus, the receiver is able to select the chrominance, sound and luminance signal portions by using appropriate frequency response networks or filters and thereafter perform individual processing thereon. Unfortunately, the frequency selection process used in most television receivers results in the loss of substantial amounts of information or image content. Perhaps the most notable loss occurs in the video or luminance information which is severely bandwidth limited as a result of the separation of chrominance and sound information. While these losses have been recognized as less than desirable, the basic filtering processes used in most television receivers has made improvement difficult or impractical. Many receivers use analog glass delay line comb filters to separate luminance and chrominance information in the shared frequency spectrum. Since glass delay lines do not provide accurate delay, factory alignments are needed to accurately separate luminance and chrominance signals.
One of the approaches contemplated by practitioners in the art seeking to improve the recovery of information at the receiver is found in the use of digital signal processing rather than the more pervasive presently used analog signal processing. Several advantages are provided by digital signal processing. For example, information processing techniques which require memory for temporary storage of information are facilitated in a digital environment due to the ease with which memory may be achieved. The combination of memory and the manner in which signals may be "clocked out" from them in digital systems make signal delays relatively easy to perform in a digital signal environment. Also, generally speaking, digital systems have evolved to a level of sophistication in which many digital systems have proven to be more economical to manufacture and have required fewer adjustments than their corresponding analog systems.
One portion of video processors which appears promising as an application of digital processing is the luminance signal delay. This need for luminance signal delay is well-known and arises due to the "slower" travel of the chrominance information through the narrow bandwidth chrominance channel relative to the "faster" travel of the luminance information through the wide bandwidth luminance channel. To make chrominance and luminance signals coincident the luminance is delayed.
The most common analog system for luminance delay makes use of either a transmission line of the necessary length or a lumped-constant approximation thereof. In either case, the input and output circuits of the delay element or "delay line" must provide a correct impedance match to avoid information loss or distortions such as transmission line "ringing".
In a digital system, however, signal delay may be obtained in a more straight-forward and reliable manner through the use of a memory which receives the to-be-delayed signal. Delay is obtained by controlling the timing of the clock signal which transfers the signal from the memory. Unfortunately, simply applying this straight-forward approach alone results in a prohibitively expensive and complex delay system.
To understand this, several fundamentals of digital signal processing systems must be recalled. For example, a fundamental bandwidth limitation is imposed upon digital processing circuits by the sample or clock rate which the system uses. Generally speaking, the sample or clock rate must be greater than twice the highest frequency signal component being processed. Unfortunately, the use of high frequency sample or clock rates often results in dramatically increased system complexity which in turn increases costs. Moreover, in broadcast formats such as the above-mentioned NTSC or PAL systems, the received information is analog information and thus the us of digital circuit processing thereon requires that the signals be converted from analog-to-digital signals. The signal must be bandwidth limited by a filter such that the highest frequency signal at the input of the analog-to-digital converter is less than half the sampling clock rate. The low pass filtering of the signal limits the high frequency components of the analog signal being converted. Avoiding such losses while processing high frequency information requires more complex (and costly) converters operated at high sample frequencies. In addition, due to higher sampling frequency, more samples require the delay which increases the amount of memory needed. Thus, the combination of long signal delay (large memory) and high clock frequency rates together with more complex high clock rate converters has, to date, made digital delay systems a high cost element of video processors.
There remains, therefore, a need in the art for a practical, cost-effective system which provides the advantages of digital electronic circuit processing for luminance delay while avoiding excessive costs and complexity.
Accordingly, it is a general object of the present invention to provide an improved video processor. It is a more particular object of the present invention to provide a cost effective digital luminance delay for use within a video processor.